Perovskite-type composite oxides each supporting a noble metal such as Pt (platinum), Rh (rhodium), or Pd (palladium) have been known as three-way catalysts which can simultaneously clean up carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) contained in emissions. Such perovskite-type composite oxides are represented by a general formula: ABO3 and enable the supported noble metal to satisfactorily exhibit its catalytic activity.
These perovskite-type composite oxides, however, undergo grain growth, thereby to have a decreased specific surface area at a high temperature of about 1000° C. The resulting catalysts can only contact with exhaust gas components in a shorter time and exhibit remarkably decreased catalytic performance in an operating environment at a high space velocity as in the case of exhaust gas purifying catalysts for automobiles.
Accordingly, various attempts have been proposed to increase their thermostability by allowing such a perovskite-type composite oxide to be supported by a thermostable composite oxide containing Ce (cerium) and/or Zr (zirconium) and, for example, the use of Ce0.8Zr0.2O2 and Ce0.65Zr0.30Y0.05O2 as the thermostable composite oxide has been proposed in Laid-open (Unexamined) Patent Publications No. Hei 5-31367, No. Hei 5-220395, No. Hei 5-253484, No. Hei 6-210175, No. Hei 7-68175, and No. Hei 7-80311.
Laid-open (Unexamined) Patent Publication No. Hei 5-31367 discloses the method comprising adding an aqueous solution containing nitrates of metal constituting a perovskite-type composite oxide in a predetermined stoichiometric ratio to thermostable composite oxide powders, drying the mixture at about 100° C. for 5 to 12 hours and further baking at 700° C. to 800° C. for 3 to 10 hours, thereby to support the perovskite-type composite oxide by the thermostable composite oxide.
Even the perovskite-type composite oxide is supported by the thermostable composite oxide containing Ce and/or Zr, however, it is difficult to secure sufficient thermostability of the perovskite-type composite oxide.